Coaxial broadband frequency multiplier employing varactor diode



COAXIAL BROADBAND FREQUENCY MULTIPLIER EMPLOYING VARACTOR DIODE Filed April 7. .1961

mnu J INVENTORS DAV/D l?, LUDWIG AV/D A. SHERMAN /A Tg GRA/EY Y B w S NN N\ mzmmg mmwm h n w@ h mw NQ J E S United States Patent O 3,194,976 COAXIAL BROADBAND FREQUENCY MULTI- PLIER EMPLOYlNG JARACTOR DIODE David R. Ludwig, Randolph, and David A. Sherman, Sudbury, Mass., assignors to General Electronics Laboratories, Inc., Cambridge, Mass., a corporation of Massachusetts Filed Apr. 7, 1961, Ser. No. 101,441 5 Claims. (Cl. 307-885) This invention relates to passive frequency multipliers and more particularly to coaxial broadband frequency multipliers utilizing nonlinear voltage variable circuit elements. By broadband as herein used is meant a capability of an .operating frequency range equal to at least ten percent Iof the mean operating frequency. The term passive as herein used is meant a capability for operat-ion without source of energy other than input signal energy of the signal to be multiplied as distinguished from the need for outside power supplies such as required by electronic tubes `which are herein referred to as structures which are active in their operation.

In the input frequency range below i000 megacycles it becomes impractical `to create frequency multipliers constructed with conventional rectangular waveguide due to prohibitively large physical size required. Also because of inherent capacitive loading and other limitations at these high frequencies, it becomes impractical and unesirable to use active multiplier devices such as triodes as distinguished from passive devices such yas voltage variable capacitors, such as varactors. The tenm varactor is a generic term in common usage identifying ak species `of commercially available PN junction semiconductor diodes having very low loss characteristics and variable capacitance which depends upon the voltage across the junction. The loss characteristics of a varactor are in the order of 100 times vsmaller than conventional junction diodes. y

A primary object of the presen-t invention is the pro-k vision of a coaxial broadband frequency multiplier which is passive in its operation.

Another object is the provision of a coaxial broadband frequency multiplier vwhich is highly efficient in its operation, that is, it has a relatively high ratio of ou-tput signal energy level to input signa-l energy level. y

And a further object is the provision of a coaxial broadband frequency multiplier which is extremely long lived.

Another object is the provision of a coaxial broadband frequency multiplier which incorporates the reliability of solid state harmonic frequency generating structures.

And a still further object is the provision of a coaxial broadband frequency `multiplier which has relatively high power output capabilities.

And another object of the present invention is the provision of a coaxial broadband frequency multiplier which is relatively compact and rugged in its construction capable of withstanding impact .and vibration without malfunction.

kAnd a still further object is the provision of a coaxial broadband frequency multiplier which lends itself to construction from generally availabley commercial components and requires relatively lit-tle adapting fabrication.

These objects, features and advantages are achieved generally by providing ya signal input circuit, a non-linear voltage variable capacitor, and an output circuit for a harmonic of the input signal frequency, the signal input circuit being arranged to excite the voltage variable capacitor over a selected frequency band and the output circuit being arranged to pass a selected harmonic of all excitation frequencies within said band.

By making theknon-linear voltage variable capacitor in p 3,194,976 Patented July 13, 1965 ICC the form of a varactor, a passive, highly efficient and long lived frequency multiplier is thereby achieved.

By providing a coaxial connector with the varactor in series with the center conductor in the connector, a desirably separable component of the overall combination is thereby achieved.

By using a bulkhead receptacle having a female center conductor contact embedded in a glass bead in one end of a plug body of the coaxial connector and carrying a varactor holder, a fixed reference location for the varactor is thereby achieved.

By providing an axially slidable male contact center conductor member in a dielectric spaced sleeve in the other end of the plug body, and with provision for holding the other terminal of the varactor diode, a secure self aligning mounting arrangement for the varactor in the co- -axial connector is thereby achieved.

By providing an internal conductor sleeve fixed in a resilient dielectric bushing surrounding the varactor and having an internal diameter whose ratio with respect to the outside diameter of thevaractor is substantially the same as that of the corresponding coaxial transmission line dimension ratio, substantially constant impedance is thereby retained.

By providing recessed and slotted end portions in the male and female contacts in the coaxial connector for .receiving the respective terminals of the varactor diode, suitable spring holders and electrical contacts for holding the varactor diode electrically and physically in place, thereby with the associated elements achieves a desirable varactor holder assembly.

By providing a low-pass filter in the signal input circuit which passes only the fundamental frequency, with the filter an electrical distance from the varactor such that a voltage minimum is reflected at the varactor by the filter for the desired output harmonic and for other harmonics below the desired output harmonic, a structure whereby harmonics higher than the fundamental frequency are prevented from reaching the sign-al source from the vai`- actor and in which maximum input signal power reaches the varactor is achieved. n

'By providing an impedance matching element interposed between the low-pass filter and signal source, suitable isolation of the signal source from the subsequent circuitry is thereby achieved so as to permit use of the present invention even where signal sources affected by changing load conditions are used.

By providing a bandpass filter at the output side of the varactor arranged to pass only the band of desired output frequencies, all undesired outputs may be prevented from reaching Ithe load.

Ey providing an electrical length of transmission line between the varactor and the band-pass output filter such that the impedances (reflected from this band-pass filter at the fundamental frequency and at the harmonic frequencies below the desired output harmonic frequency) are proper to simultaneously resonate with the average capacitance of the varactor at ea-ch of these above mentioned frequencies, maximum output power over the selected output frequency band of the frequency multiplier is thereby achieved. For multipliers of order greater than two, this electrical length condition results in the production of an idler current through the varactor at harmonic frequencies of the input below the desired output harmonic. These configurations on the input and output circuitry provide a structure for achieving maximum operating efficiency of the frequency multiplier.

By making the band-pass filter in the form of two series coupled filters, namely a high pass filter and a low pass filter, readily available commercial filters may thereby be used;

These features, objects and advantages will be better understood from the following description taken in connection with the accompanying drawings of preferred embodiments of the invention and wherein:

FIG. l is a block diagram of a frequency multiplier constructed in accordance with the present invention;

FIG. 2 is a plan view of a frequency tripler embodiment of the invention;

FlG. 3 is a cross section taken on line 3-3 of PEG. 2 to more clearly show construction of a portion of the embodiment shown in FIG. 2.

Referring to FIG. 1 in more det-ail, a frequency multiplier made in accordance with the present invention is illustrated generally by the numeral lt?. The frequency multiplier lil is fed by a conventional oscillator l2 through an impedance matching network ld coupled by a coaxial connector lo to the oscillator l?. and a coaxial connector l5 to a conventional low-pass filter Ztl whose upper frequency marks the upper frequency of the selected band of frequencies from the oscillator l2 to be fed to the frequency multiplier lt?.

The low-pass filter 29 is coupled to a varactor holder assembly 22, to be hereinafter further described in connection with FIG. 3, by a length 2.4 of a coaxial transmission line such that a voltage minimum is reflected from the low-pass filter 2u to the varactor holder assembly 2?; for the selected output harmonic and for other harmonic frequencies below the selected harmonic of the input signal frequency band. This proper length of line 24!- may be found with the aid of a standing wave detecting slotted transmission line by which voltage standing Wave patterns of the low-pass filter are determined at the harmonic frequencies. Having found the desired length of transmission line 24 from the slotted transmission line measurements, the standing wave detecting transmission line is replaced with an actual length of coaxial transmission line 24 which is then empirically adjusted to compensate for discontinuity in connectors, differences in the slotted line and actual coaxial transmission line 2d, and lead inductance to the varactor holder assembly 22, to produce thereby maximum power output at the selected harmonic.

In similar manner, a coaxial transmission line having a length 26 is used to couple a varactor holder assembly 22 to a high pass filter 28 so as to reflect to the varactor holder assembly 22 an impedance proper to resonate with the average varactor impedance at each of the harmonic frequencies below the desired harmonic and at the fundamental frequency. The high pass filter 28 is coupled by a coaxial transmission line 3) to a lowpass filter 32 having a signal output coupling 34 to a desired use device or load such as a mixer (not shown) or other desired equipment, as a frequency source for the equipment.

The coaxial transmission line Sti has an electrical length 36 which is empirically adjusted for maximum power output of the separate filter components 2S and 3:2 to thereby permit the use of commercially available filters in place of a single lumped band pass output filter which would necessarily be custom built at substantially greater expense than the construction herein described.

ln the operation of the FIG. 1 embodiment, input signal frequencies from the oscillator l2 are fed through the coaxial coupling 16 and impedance matching network 14, and coaxial coupling 18, through the lowpass lter Ztl where frequencies above the selected band of input frequencies are attenuated. The desired frequencies on the input band are then fed from the low-pass filter Ztl through the length of coaxial transmission line 2d to the varactor holder assembly Z2 so as to excite the varactor in the varactor holder assembly 22 to produce harmonics of the input frequency signals. These harmonics are fed from the varactor holder assembly Z2 through coaxial transmission line to the high-pass filter 23 where frequencies below the desired output frequency band are reflected. The frequencies above the reflected frequencies are passed through the coaxial transmission line to the low-pass filter ST. which prevents the passage of frequencies higher than those selected for the output so as to pass to the connector output 3dthe selected harmonics in the band of frequencies selected for the frequency multiplier it).

Referring more particularly to FIG. 2, there is illustrated a frequency tripler which is a specific embodiment of the general construction shown in FlG. l. ln the frequency tripler in FlG. 2, a low-pass filter 3S which may for input frequencies of 700 megacycles be of the commercially available type identified as model number LA-lST, available from Microlab Corp., 570 W. Mt. Pleasant Avenue, Livingston, NJ.

The low pass filter 3S may be coupled through a type BNC connector il to a type RGSS/U coaxial transmission line d?. to a type N connector lil into a varactor holder assembly do, to be hereinafter further described in connection with FG. 3. The coaxial line d?, has an electrical length 4S described above in connection with transmission line 24.

The varactor holder assembly do is coupled through an adapter which will be described in connection with FG. 3 to a high-pass filter which is coupled through a type BNC connector Si@ to a type RGSS/ U coaxial transmission line 52 and a type TNC male connector 5d into a type T NC female and type N male adapter SI5 into a type N right angle elbow 5S and into a low-pass filter dit through a type N connector d2 into the output type N connector 64. The high and low pass filters 49 and 69 may be of the type identified as model F-40 and LTP respectively, available from RLC Electronics lnc., 805 Mamaroneck Ave., Mamaroneck, New York.

It should be undertsood that the references herein made to specic commercially available components are for illustration of an important feature of this invention which is that of its adaptability of construction with readily available elements. These references to specific commercially available components are not intended as a limitation since other suitable components may also be used.

Referring to FIG. 3 in more detail, the varactor holder assembly 46 is a bulkhead type receptacle having a jack body 66 with a female contact d8 running axially thereof as a center conductor and rigidly fixed in place by a fused glass bead 70. The center conductor 68 has integrally therewith at its other end a recessed varactor holder '72 carrying axial slots 74 which permit sufficient spring action in the varactor holder 72 to electrically and mechanically engage and hold in place a varactor 76 by one of its terminals in a fixed reference position. Concentrically about the varactor 76 is an electrically conductive sleeve 78 which forms the outer coaxial conductor at the varactor '76. The sleeve 7 tl is held rigidly in place between a shoulder 8l) in a plug body 82 and a tapered end 84 of the jack body 65. A Teflon alignment gasket 86 holds the sleeve 7S in place during disassembled condition of the varactor holder assembly 46 and provides suiiicient resilience to properly compensate for tolerances at the tapered end surface 84 to insure proper assembly.

The plug body 82 has a dielectric alignment bushing 88 of such material as Teflon which slidably retains a combined male contact 90 and a varactor holder 92 similar to the varactor holder 72 and in proper alignment therewith for engaging the other terminal of the varactor 76. A rubber seating bushing 94 is provided at the end of the plug body 82 for achieving a water tight seal when by means of the knurled nut 96 it is screwed onto the threaded end of a jack body 93 on the end of the high pass filter 49, at which time provision for contact with an outer conductor 100 fixed to the end of jack body 82 provides coaxial line continuity in conventional manner of coaxial couplings.

In the assembly of the bulkhead receptacle or varactor holder assembly 46, the jack body 65 is screwed into the end of they plug body 82 until tirm engagement is made with the internal sleeve 7S at the tapered surface 84. A nut 192 is then tightened against the plugbody 82 to lock the assembly iirmly in place.

The internal diameter of thesleeve 78 is proportioned with respect to the diameter of the varactor 7d and varactor holders 72 and 92 to retain impedance continuity with that of the adjoining coaxial transmission line.

This invention is not limited to the particular details of construction and operation herein described as equivalents will suggest themselves to those skilled in the art.

What is claimed is:

1. In combination, a coaxial input circuit means for oscillatory electric signals, voltage variable capacitor means having a nonlinear lresponse characteristic to voltage excitation in a coaxial housing, coaxial output circuit means, the input circuit means being coupled to said voltage variable capacitor housing for passing for excitation of said voltage variable capacitor means by said f oscillatory signals over a selected frequency range and including a coaxial coupling to the voltage variable capacitor yof a length to reflect a voltage minimum to the voltage variable capacitor means at a selected harmonic of excitation frequencies, and the output circuit means being coupled to said voltage variable capacitor housing for passing from said voltage variable capacitor means a selected harmonic of all excitation frequencies within said selected range.

2. The combination as in claim l wherein the voltage Variable capacitor means includes a varactor diode,

3. In a coaxial broadband frequency multiplier for producing an output signal which is a selected harmonic of an oscillatory electric input signal, the combination of a coaxial cable connector having an external and internal conductor, low-pass signal input coaxial lter means coupled to the connector with an electrical length of coaxial cable between the lilter and connector such as to effect a voltage minimum at the varactor for the selected output harmonic and the harmonics below that of the selected E output harmonic, and signal output coaxial iiiter means coupled to the connector for passing a band of frequencies at said selected harmonic of the input frequency.

4. The combination as in claim 3 wherein a coaxial cable couples the signal output lter means to said connector and is of an electrical length to reiiect impedances at the input signal frequency and harmonic frequencies below the desired output such that the average capacitance of the varactor will be tuned at these frequencies thus allowing maximum current iiow through the varactor at these frequencies.

5. in combination, a varactor diode having two terminals, an outer conductor about said diode, an inner conductor extending from each of the varactor terminals, means coupled to the inner and outer conductor on the side of one of the varactor terminals for applying oscillatory electric excitation to the varactor and including a transmission line from said means to said varactor of an electrical length to reflect a voltage minimum to the varactor at a selected harmonic of the applied excitation, and means coupled to the inner and outer conductors on the side of the other of the varactor terminals for isolating a selected harmonicy of the applied oscillatory excitation emanating from the varactor.

References Cited by the Examiner UNITED STATES PATENTS 2,785,308 3/57 Stahl 331-76 2,914,740 ll/59 Blonder S33-97 2,918,638 12/59 Cattoi et al 333-97 2,964,649 12/60 Vance 307--885 2,982,922 5/61 Wilson 328-16 X OTHER REFERENCES 1959 Solid State Circuit Conference, Digest of Technical Papers, pages 82, 83, Feb. 13, 1959, Session Vll.

ARTHUR GAUSS, Primary Examiner.

JOHN KOMINSKI, Examiner. 

1. IN COMBINATION, A COAXIAL INPUT CIRCUIT MEANS FOR OSCILLATORY ELECTRIC SIGNALS, VOLTAGES VARIABLE CAPACITOR MEANS HAVING A NON-LINEAR RESPONSE CHARACTERISTIC TO VOLTAGE EXCITATION IN A COAXIAL HOUSING, COAXIAL OUTPUT CIRCUIT MEANS, THE INPUT CIRCUIT MEANS BEING COUPLED TO SAID VOLTAGE VARIABLE CAPACITOR HOUSING FOR PASSING FOR EXCITATION OF SAID VOLTAGE VARIABLE CAPACITOR MEANS BY SAID OSCILLATORY SIGNALS OVER A SELECTED FREQUENCY RANGE AND INCLUDING A COAXIAL COUPLING TO THE VOLTAGE VARIABLE CAPACITOR OF A LENGHT TO REFLECT A VOLTAGE MINIMUM TO THE VOLTAGE VARIABLE CAPACITOR MEANS AT A SELECTED HARMONIC OF EXCITATION FREQUENCIES, AND THE OUTPUT CIRCUIT MEANS BEING COUPLED TO SAID VOLTAGE VARIABLE CAPACITOR HOUSING FOR PASSING FROM SAID VOLTAGE VARIABLE CAPACITOR MEANS A SELECTED HARMONIC OF ALL EXCITATION FREQUENCIES WITHIN SAID SELECTED RANGE. 